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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Short-term investment:      
Trading securities $ 208.0 $ 486.5  
Weighted average interest rate (as a percent) 4.50% 0.30%  
Available-for-sale $ 0.3 $ 0.4  
Weighted average interest rate (as a percent) 0.80% 0.70%  
Total $ 208.3 $ 486.9  
Trading:      
Interest earned 4.4 1.4  
Unrealized gain (loss) at the end of the period   0.1  
Available-for-sale:      
Interest earned 35.0 7.2 $ 19.2
Investment redeemed 0.1 0.2  
Contractual maturities of available-for-sale debt securities      
Due after 10 years 0.3 0.4  
Total debt securities 0.3 0.4  
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1